1. Field of the Invention
The present invention relates generally to spread spectrum communication systems and more specifically to a spread spectrum receiver use in a mobile communication system that can establish synchronization in a short period of time.
2. Description of the Related Art
In a spread spectrum communication system, a rake receiver is known, which consists of a plurality of rake fingers and a rake combiner that combines the outputs of the rake fingers. Each rake finger includes a despreading circuit that detects correlation between a received spread spectrum signal and a despreading code (or pseudonoise sequence) of a bit sequence identical to the despreading codes of other rake fingers. The despreading code of each rake finger is offset with respect to the reference timing of the code such that the offset values of all rake fingers are incrementally different by a chip interval. The timing offset values of all rake fingers are successively varied in search of the correct receive timing of a transmitted signal so that in one of the rake fingers the despreading code will eventually coincide with the reference timing of the transmitted signal. The process used in this timing search is a multi-window technique, which ensures a quick search.
In the conventional CDMA (code division multiple access) mobile communication systems, the multi-window technique is employed for quickly establishing timing with the control channel. However, when timing is established with the control channel, it is the usual practice to select a predetermined one of the rake fingers and to set the selected rake finger with the PN code of an assigned communication channel. The selected rake finger is then set with a timing offset value that is determined from the timing established with the control channel. Thus, when a connection is established, the selected rake finger can immediately establish synchronization with a speech signal transmitted on the assigned communication channel.
Since the multi-window mode of operation continues only for a short, call-setup time, economy can be achieved by sharing the multi-window rake receiver configuration among calls and assigning single-window despreading circuits to individual calls. However, if the receiver loses track of the correct timing, it initiates a hunt for the lost signal using the single search window. Such a misalignment condition can occur if the mobile station rapidly moves around or if the field strength of the speech signal sharply drops as a result of fading. Be cause of the single window se arch, it takes a long time to re-establish synchronization.
It is therefore an object of the present invention to provide a receiver and method for receiving a spread spectrum signal that prevents timing misalignment during a call.
According to a first aspect of the present invention, there is provided a spread spectrum receiver comprising a plurality of rake fingers for receiving a spread spectrum signal, each rake finger including despreading circuitry for despreading the spread spectrum signal with a despreading code, a rake combiner for combining output signals of the rake fingers, and control circuitry. The control circuitry has the functions of setting despreading codes of a first bit sequence to the rake fingers at respective timing offsets of incremental values so that there exists a timing difference of one chip interval between successive rake fingers, and successively varying the respective timing offsets to receive a call processing signal. Responsive to an output of the rake combiner indicating that the call processing signal is received, the control circuitry determines the timing offset of the received call processing signal, and sets despreading codes of a second bit sequence to the rake fingers at respective timing offsets based on the determined timing offset in order to receive an information signal. The control circuitry may be arranged to successively vary the respective timing offsets o f the despreading codes of the second bit sequence in response to an output of the rake comb iner indicating that the information signal is lost.
According to a second aspect, the present invention provides a spread spectrum receiver comprising a plurality of rake fingers for receiving a spread spectrum signal, and a rake combiner for combining output signals of the rake fingers. Each rake finger includes a plurality of despreading circuitry for despreading the spread spectrum signal with a plurality of despreading codes of identical bit sequence, a plurality of signal-to-interference detecting circuitry associated respectively with the plurality of despreading circuitry for determining the respective signal-to-interference ratios of output signals of the plurality of despreading circuitry, and selecting circuitry for selecting one of the output signals of the despreading circuitry having a maximum signal-to-interference ratio as an output signal of the rake finger. Control circuitry has the functions of setting despreading codes of a first bit sequence to all despreading circuitry of all rake fingers at respective timing offsets of incremental values so that there exists a timing difference of one chip interval between successive ones of all despreading circuitry, and successively varying the respective timing offsets of all rake fingers to receive a call processing signal. The control circuitry is responsive to an output of the rake combiner, indicating that the call processing signal is received, for determining a timing offset of the received call processing signal, and setting despreading codes of a second bit sequence at respective timing offsets to all despreading circuitry of all rake fingers based on the determined timing offset to receive an information signal. The control circuitry may be arranged to successively vary the respective timing offsets of the despreading codes of the second bit sequence set in the despreading circuitry of all rake fingers in response to an output of the rake combiner indicating that the information signal is not received.
According to a third aspect, the present invention provides a method of receiving a spread spectrum signal by using a plurality of rake fingers and a rake combiner, wherein each of the rake fingers receives a spread spectrum signal and despread the spread spectrum signal with a despreading code of identical bit sequence, and the rake combiner combines outputs of the rake fingers. The method comprises the steps of setting despreading codes of a first bit sequence to the rake fingers at respective timing offsets of incremental values so that there exists a timing difference of one chip interval between successive rake fingers, successively varying the timing offsets of the despreading codes, determining a timing offset in response to an output of the rake combiner indicating that a call processing signal is received, and setting despreading codes of a second bit sequence to the rake fingers at respective timing offsets of incremental values based on the determined timing offset to receive an information signal.
According to a fourth aspect, the present invention provides a method of receiving a spread spectrum signal by using a plurality of rake fingers for receiving a spread spectrum signal, and a rake combiner, wherein each rake finger includes a plurality of despreading circuitry for despreading the spread spectrum signal with a plurality of despreading codes of identical bit sequence, a plurality of signal-to-interference detecting circuitry associated respectively with the plurality of despreading circuitry for determining the respective signal-to-interference ratios of output signals of the despreading circuitry, and selecting circuitry for selecting one of the output signals of the despreading circuitry having a maximum signal-to-interference ratio, wherein the rake combiner combines the selected signals of the rake fingers. The method comprises the steps of setting despreading codes of a first bit sequence to all despreading circuitry of all rake fingers at respective timing offsets of incremental values so that there exists a timing difference of one chip interval between successive ones of all despreading circuitry, successively varying the respective timing offsets of all rake fingers to receive a call processing signal, determining a timing offset in response to an output of the rake combiner indicating that the call processing signal is received, and setting despreading codes of a second bit sequence to all despreading circuitry of all rake fingers at respective timing offsets of incremental values based on the determined timing offset to receive an information signal.